A speech codec for VoIP (Voice over IP) use is required to have high packet loss tolerance. It is desirable for a next-generation VoIP codec to achieve error-free quality even at a comparatively high frame loss rate (for example, 6%).
In the case of CELP speech codecs, there are many cases in which quality degradation due to frame loss in a speech onset portion is a problem. The reason for this may be that signal variation is great and correlativity with the signal of the preceding frame is low in the onset portion, and therefore concealment processing using preceding frame information does not function effectively. Alternatively, the reason may be that in a frame of a subsequent voiced portion, an excitation signal encoded in the onset portion is actively used as an adaptive codebook, and therefore the effects of loss of an onset portion are transmitted to a subsequent voiced frame, tending to cause major distortion of a decoded speech signal.
In response to the above kind of problem, a technology has been developed whereby encoded information for concealment processing when a preceding or succeeding frame is lost is transmitted together with current frame encoded information (see Patent Document 1, for example). With this technology, it is determined whether or not a preceding frame false signal (or succeeding frame false signal) can be created by synthesizing a preceding frame (or succeeding frame) concealment signal by repetition of a current frame speech signal or extrapolation of a characteristic amount of that code, and comparing this with the preceding frame signal (or succeeding frame signal), and if it is determined that creation is not possible, a preceding subcode (or succeeding subcode) is generated by a preceding sub-encoder (or succeeding sub-encoder) based on a preceding frame signal (or succeeding frame signal), and it is possible to generate a high-quality decoded signal even if a preceding frame (succeeding frame) is lost by adding a preceding subcode (succeeding subcode) to the main code of the current frame encoded by a main encoder.    Patent Document 1: Japanese Patent Application Laid-Open No. 2003-249957